Checking the Cessna 152 POH it says to increase takeoff distance by 15% of ground roll (makes sense due to increased friction). Also says to increase landing distance by 45% (doesn't seem to make sense). Would like to understand why.

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    $\begingroup$ It says increase landing distance by 45% in wet grass I believe. $\endgroup$ – GdD Mar 8 '18 at 16:53
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    $\begingroup$ And takeoff distance too... The most scary takeoff I remember was on a grass surface, long grass and very wet with the dew of the early morning. I thought that the runway available was enough, and that 'that of the wet grass' was just something 'in the books'... To this day, I believe that I survived that takeoff thanks to some lift from Above... $\endgroup$ – xxavier Mar 8 '18 at 17:17

The long and short of it is that wheels are designed for smooth tarmac. While grass increases friction on the wheels on takeoff (making the aircraft work harder to get up to speed), it also reduces the efficiency of the brakes, thus taking longer to slow down.

And to add to @GdD's point, grass retains moisture much better than tarmac, so in wet conditions it is considerably slippier (and will remain so for longer afterwards).

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    $\begingroup$ O.K. thanks. I normally don't use the brakes very much or hard so I didn't factor this into the equation. 45% can be quite significant. $\endgroup$ – Larry Mar 8 '18 at 17:08
  • $\begingroup$ @Larry Fair enough, it's worth remembering that grass runways can (and often are) shorter than the runway you might be used to, so don't scrimp on those brakes! $\endgroup$ – Cooper Mar 8 '18 at 17:14
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    $\begingroup$ I have to disagree: wheels are NOT designed for smooth tarmac. They are designed to work on any fairly smooth surface, whether it's tarmac, grass, concrete, dirt, or (in northern parts) a frozen lake. It's just that those different materials have different properties, which change the amount of rolling friction (or sliding, if you brake too hard). Same principle would apply if you're landing on say a paved runway with patches of snow or ice. $\endgroup$ – jamesqf Mar 8 '18 at 17:20
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    $\begingroup$ @Cooper: Sure, that's why POHs recommends increasing takeoff & landing distances on different surfaces. You probably learned something about this in basic flight training, too, just as you learned to take density altitude into account. (I'd expect the reason the base distances are quoted for pavement is simply that it's repeatable, while grass & dirt vary a lot.) $\endgroup$ – jamesqf Mar 9 '18 at 19:34
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    $\begingroup$ @Sean Without using any brakes I suppose, yes. $\endgroup$ – Cooper Jan 16 '19 at 12:30

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